It is well known in the medical arts that certain chemical components (“analytes”) in breath are correlated with certain internal physiological and pathophysiological processes or states in the body, and thus such analytes at least in theory can serve as biomarkers for detecting or assessing such processes or states. Risby et al., Current Status of Clinical Breath Analysis, Appl. Phys. (2006), for example, provides a table that lists endogenous breath analytes and the corresponding physiological origin associated with each.
Ammonia in breath has been correlated with several physiological or pathophysiological states. One involves the cellular metabolism of proteins. In the course of metabolism of proteins, the amine groups are converted into ammonia. That ammonia makes its way through the bloodstream to the lungs, where it diffuses across the alveolar membrane into the alveolar air space. As normal respiration occurs, the ammonia-containing air in the lungs is exhaled, where it can be collected and analyzed.
Breath ammonia also is correlated with certain liver pathologies. In the urea cycle, for example, the liver converts ammonia to urea and uric acid. When anomalies in the urea cycle occur, the unreacted ammonia can accumulate and lead to maladies including hepatic encephalopathy. As the ammonia accumulates in the blood, a portion of it passes into the alveolar spaces, from where it is mixed with tidal air and makes its way into exhaled breath.
Another physiological or pathophysiological state that has been correlated with ammonia involves renal dialysis. As reported, for example, in L. R. Narasimhan, William Goodman, and C. Kumar N. Patel, Correlation Of Breath Ammonia With Blood Urea Nitrogen And Creatinine During Hemodialysis, PNAS, Vol. 98, No. 8, pgs. 4617-4621, Apr. 10, 2001, breath ammonia concentration has been correlated with blood-urea-nitrogen (“BUN”) levels. BUN levels are used clinically during dialysis treatments to assess the adequacy of the dialysis process. Narasimhan et al. report that breath ammonia levels drop by around 90% during the first 30 minutes of hemodialysis and correlate well with BUN and creatinine levels. The use of breath ammonia to make this assessment allows a real time or near-real time measure, as opposed to the non-real time and only periodic testing associated with BUN measurement using blood samples.
Breath ammonia also can be used to detect and assess end-stage renal failure. As renal function decays during such failure, nephrons fail to remove nitrogen-bearing wastes from the blood. The resultant elevated levels of nitrogenous wastes, including ammonia, travel through the bloodstream and eventually pass through the lung barrier into the alveolar air, and into exhaled breath. While a healthy population may have breath ammonia levels of less than about 100 parts per billion (“ppb”), patients with end stage renal failure can reach breath ammonia levels of 13,000 ppb. David Smith, Tianshu Wangl, Andriy Pysanenkol, Patrik Španěll, A Selected Ion Flow Tube Mass Spectrometry Study of Ammonia in Mouth-and Nose-Exhaled Breath and In the Oral Cavity”; S. Davies, P. Španěl and D. Smith, Quantitative Analysis Of Ammonia On The Breath Of Patients In End-Stage Renal Failure,” Kidney Int. 52, 223-228 (1997).
Breath ammonia also has been correlated with certain forms of gastritis and gastrointestinal ulcers. U.S. Pat. No. 4,947,861, issued to Hamilton on Aug. 14, 1990 (“Hamilton”) at col. 1, for example, reports that “[i]t is a relatively recent discovery that a colony of Campylobacter pylori [(or C. pylori, now Helicobacter pylori or H. pylori)] bacteria is usually found associated with gastritis and duodenitis, and is frequently found at the sites of peptic and duodenal ulcers.” According to Hamilton, “Graham et al. reported successful results with a breath test wherein patients first ingested urea labeled with carbon-13, a stable, naturally occurring non-radioactive isotope. In the presence of urea, C. pylori produces urease, an enzyme that breaks down urea into ammonia, carbon dioxide and other products.” Some portion of the ammonia generated at the gastrointestinal site is then transported via the vascular system to the lungs, where it enters the alveolar air space and is exhaled in the breath. (See, also, U.S. Pat. No. 5,179,052 to Ito et al.)
The sensing of breath ammonia, however, presents several challenges. One such challenge is attributable in part to the low levels or concentrations of the ammonia typically in breath, even under pathophysiological states. Another involves the fact that breath ammonia is accompanied by water or humidity, wherein the water is in great excess. Ammonia is highly soluble in water. Given the extremely close molecular weight of the two (17 for ammonia versus 18 for water), they present challenges in separation, detection, masking, etc. This molecular weight similarity makes their fractionation products virtually indiscernible with a mass spectrometer.
A number of technical approaches have been used to sense ammonia in breath. One such technique involves the use of gas chromatography (“GC”). This technique is limited, however, particularly where field or home use is required or desirable, in that GC equipment is relatively large, expensive, and requires trained technicians to operate. These factors confine GC largely to laboratory or hospital settings. Notwithstanding this, generally-accepted GC methods have been relatively unsuccessful for sensing ammonia in breath because of the presence of relatively high concentrations of water and the low concentrations of ammonia.
Newer methods, such as selected ion flow tube-mass spectrometry (“SIFT-MS”), are capable of detecting ammonia at the low levels present in breath in essentially real time. Such instruments, however, are expensive, typically costing in excess of $200,000, they are relatively inaccessible given how few are in existence, they weigh over 200 kilograms (“kg”), and they currently do not have regulatory approval for breath analysis. Again, they are highly unsuited for field or home use.
Another approach for sensing of breath ammonia has involved the use of pH indicators. Such indicators have been used for many decades in the detection of various acidic and basic gases in the ambient air. U.S. Pat. No. 3,131,030 to Grosskopf, for example, discloses impregnating polystyrene beads with methyl violet for detection of the basic gas hydrazine. U.S. Pat. No. 3,350,175 to McConnaughey discloses impregnating clay spheres with thymol blue and oxalic acid, which are then adsorbed to 60 to 80 mesh glass spheres for the detection of ammonia. McConnaughey reports a detection limit of just under 250 parts per million (“ppm”), which is substantially less sensitive than the levels typically required for sensing ammonia in breath. U.S. Pat. No. 4,201,548 to Katsuki discloses impregnating a polyethylene tape with bromocresol green. Katsuki reports the ability to detect ammonia at levels near 500 ppb in liquid samples, but not in breath. U.S. Pat. No. 4,947,861 (Hamilton) discloses using a Gastec® ammonia tube (cresol red pH indicator with a 1,000 ppb preferred detection limit) connected to an Ascarite II® (registered trademark of Authur H. Thomas Co.) desiccant to detect ammonia produced by H. pylori bacteria in the digestive tract.
In view of the foregoing, there is a clear need for improved devices and methods that can enable one to sense breath ammonia down to the concentration ranges in which ammonia is present in breath during various physiological and pathophysiological states, e.g., at levels of 0.2 ppm and below. Such devices and methods preferably would be able to sense ammonia in breath samples that comprise water, carbon dioxide and other chemical constituents, and still have the sensitivity and discrimination capabilities to accurately and reliably measure the ammonia.
There also is considerable advantage in providing such breath analysis devices and methods that can accurately and reliably sense ammonia in a clinical, field or patient home setting, which implicates a need for small or portable, cost-effective devices and components, and methods that lend themselves to such applications.